P-Channel Power Mosfet Help

[behrendfry]

I have a PCB with a IRFD9120 (P-Channel Power Mosfet) and I am having some trouble understanding how it works. The PCB board has to inputs that each connect to a filter, and then one input goes to the drain and the other input goes to the source, this input also goes to the TC622 Temperature Sensor. The OUT, pin 2 of the TC622 is connected to the gate of the mosfet. This circuit is used to keep a bottle at a constant temperature. So when the TC622 detects the temperature to be below the SET Temperature the OUT output is high which will send a voltage to the gate of the mosfet. The drain is also connected to the coil that wraps around the bottle and when current flows through the coill the bottle heats up. I know that for drain current to flow the gate voltage has to be increased towards the drain voltage, which is why the gate is connected to the TC622 so when it receives a high signal this gate voltage increases to the drain voltage allowing current to flow. So here is my issue, I do not know the inputs to the circuit but I do know the input that goes to the SOURCE and TC622 is the same and the TC622 can only handle between 4-18VDC so the input that goes to the drain must be between 4-18Vdc also right??? Does anyone have any suggestions on how I can build a quick test circuit using my Mosfet and TC622 and verify that when the TC622 OUT goes high that the Drain current will flow. Hence some kind of visual device, like an LED that will let me know that the MOSFET is operating properly. Any suggestions would be helpful, maybe my assumptions about the circuits inputs are wrong so any help would be great.

 

[Russlk]

One normally thinks of "high" as positive but the P-MOSFET wants to see negative at the gate. Perhaps that is your problem?

 

[eblc1388]

For a P-Channel MOSFET to conduct, its gate voltage must be more negative than the source voltage(i.e. more towards the 0V rail).

In the schematic, one single resistor Rset would set the operating temperature value. If the temperature sensed is below the set point temperature, the /OUT pin is LOW. This will turn on P-Channel MOSFET and the heater.

When the set point temperature is reached, /OUT goes HIGH and MOSFET turns OFF, cutting off current to heater.

A LED is added in the schematic to show both the correct operation of the controller and MOSFET.